1. Field of the Invention
The invention relates to a method of producing salcomine in which ethylene diamine is reacted with approximately twice the molar amount of salicylaldehyde, relative to ethylene diamine, and with a cobalt salt using a solvent in a liquid reaction medium at temperatures of 60.degree.-150.degree. C.
2. Description of the Related Art
Salcomine, that is, N,N-bis-salicylidene ethylene diimine cobalt (II) of the formula ##STR1## is a very suitable catalyst for the oxidation of alkylated phenols to the corresponding alkylated p-benzoquinones, especially for the oxidation of 2,3,6-trimethylphenol to 2,3,5-trimethyl-p-benzoquinone, which can be readily converted by hydrogenation into 2,3,5-trimethylhydroquinone. For its part, trimethylhydroquinone is very interesting as an intermediate [i. product] for vitamin E.
In known methods for the production of salcomine the reaction of ethylene diamine and salicylaldehyde with a cobalt (II) salt takes place e.g. in aqueous or aqueous-organic solution.
The presence of a buffer (e.g. aqueous acetic acid sodium acetate solution or an alkanol-pyridine mixture) is also recommended for a better dissolving of the Schiff base of salicylaldehyde and the diamine. The precipitating salcomine must finally be filtered off, washed several times and dried.
In particular, an expensive workup requiring filtration, several washings and drying is disadvantageous in this method and in other known methods which are collated in D. Aymes, M. R. Paris, Bull. Soc. Chim. Fr. 1976, 1717-21 and D. Aymes, M. R. Paris, J. Chem. Educ., 66 (1989), 854-56.
In addition, the salcomine obtained is not always of a uniform quality as regards its oxidation action.
The improvements suggested by Aymes et al. in the cited literature of using benzene, chloroform or methylene chloride and the cobalt acetylacetonate soluble therein as solvent in the production of salcomine have the disadvantages of low yields of salcomine and the use of expensive cobalt compounds and solvents which are somewhat hazardous to health.
DE 33 02 498 suggests a method of producing salcomine by reacting 2 moles salicylaldehyde with 1 mole ethylene diamine and a cobalt salt in a liquid reaction medium in which the salicylaldehyde is reacted with the ethylene diamine in a linear or cyclic carboxylic acid amide substituted on the nitrogen as solvent and the reaction mixture obtained is reacted at temperatures of 60.degree. to 150.degree. C. with cobalt carbonate or cobalt hydroxide carbonate 2 CoCO.sub.3.3 Co(OH).sub.2.
In particular, dimethylformamide or N-methylpyrrolidone are examples of solvents according to DE 33 02 498. Although yields of salcomine of up to 99% are reported in the patent under discussion, these high yields were not able to be verified. Contrary to the data in the patent, the yields in reference tests were only approximately 60% for cobalt hydroxy carbonate (see below Reference Example 1 which is approximately equal to example 3 in DE 33 02 498) or approximately 90% when using cobalt carbonate (example 1 in DE 33 02 498). In addition, in the method according to DE 33 02 498, the use of solvents injurious to health appears hazardous.
Methods of producing chiral catalysts similar to salcomine are known from WO 93/03838. However, exclusively trivalent metal complexes are disclosed there.
DE-A-42 38 076 also describes a compound similar to salcomine. However, the solubility behavior of Co(5-NO.sub.2 -saltmen) is distinctly different from that of salcomine.